Polydienes may be produced by solution polymerization, wherein conjugated diene monomer is polymerized in an inert solvent or diluent. The solvent serves to solubilize the reactants and products, to act as a carrier for the reactants and product, to aid in the transfer of the heat of polymerization, and to help in moderating the polymerization rate. The solvent also allows easier stirring and transferring of the polymerization mixture (also called cement), since the viscosity of the cement is decreased by the presence of the solvent. Nevertheless, the presence of solvent presents a number of difficulties. The solvent must be separated from the polymer and then recycled for reuse or otherwise disposed of as waste. The cost of recovering and recycling the solvent adds greatly to the cost of the polymer being produced, and there is always the risk that the recycled solvent after purification may still retain some impurities that will poison the polymerization catalyst. In addition, some solvents such as aromatic hydrocarbons can raise environmental concerns. Further, the purity of the polymer product may be affected if there are difficulties in removing the solvent.
Polydienes may also be produced by bulk polymerization (also called mass polymerization), wherein conjugated diene monomer is polymerized in the absence or substantial absence of any solvent, and, in effect, the monomer itself acts as a diluent. Since bulk polymerization is essentially solventless, there is less contamination risk, and the product separation is simplified. Bulk polymerization offers a number of economic advantages including lower capital cost for new plant capacity, lower energy cost to operate, and fewer people to operate. The solventless feature also provides environmental advantages, with emissions and waste water pollution being reduced.
Despite its many advantages, bulk polymerization requires very careful temperature control, and there is also the need for strong and elaborate stirring equipment since the viscosity of the polymerization mixture can become very high. In the absence of added diluent, the high cement viscosity and exotherm effects can make temperature control very difficult. Consequently, local hot spots may occur, resulting in degradation, gelation, and/or discoloration of the polymer product. In the extreme case, uncontrolled acceleration of the polymerization rate can lead to disastrous “runaway” reactions. To facilitate the temperature control during bulk polymerization, it is desirable that a catalyst gives a reaction rate that is sufficiently fast for economical reasons but is slow enough to allow for the removal of the heat from the polymerization exotherm in order to ensure the process safety.
Lanthanide-based catalyst systems that comprise a lanthanide compound, an alkylating agent, and a halogen source are known to be useful for producing conjugated diene polymers having high cis-1,4-linkage contents. The resulting cis-1,4-polydienes typically have a cis-1,4-linkage of less than 99%. Molecular weight distributions vary, but are typically above 2. It is known that cis-1,4-polydienes having higher cis contents, and narrower molecular weight distributions, give a greater ability to undergo strain-induced crystallization and lower hysteresis and thus, give superior physical properties such as higher tensile strength and higher abrasion resistance. Therefore, there is a need to develop a process for producing cis-1,4-polydienes having a combination of ultra-high cis contents (greater than 99% cis) and narrow molecular weight distributions.
Unfortunately, most catalyst systems can not consistently achieve all of these properties. For example, catalysts have been developed to produce polymers with cis-1,4-linkage contents above 99%, but have broad molecular weight distributions. Furthermore, many of these catalysts use highly active, Lewis acidic chlorides, bromides, and iodides to achieve these properties, which results in excessively fast polymerization rates. This makes it very difficult to control the temperature and compromises the process safety. Fast polymerization rates and uncontrollable temperatures often lead to gel formation inside the polymerization reactor due to excess polymer formation on the walls of the reactor. In turn, the reactor must be cleaned before another polymerization can be conducted resulting in expensive delays in production.
Therefore, it is desirable to develop a bulk polymerization method for producing cis-1,4-polydienes having higher cis-1,4-linkage content and lower molecular weight distribution.